The modern development of optical fibers and semiconductor diode lasers has enabled optical signaling at very high bit rates, Examples of such optical signaling are fiber-optic communications, physical measurements, time-division multiplexing, and electrooptical sampling. However, the full potential of such systems depends upon the generation of picosecond optical pulses at multigigahertz repetition rates, the flexible synchronization of the optical pulses to an external clock, and the time translation of the optical pulses with respect to the external clock.
Such systems typically rely upon the generation of picosecond optical pulses by one of two methods, gain switching and mode locking. In gain switching, a pulsed current source directly modulates the drive current of a diode laser. Current modulation results in a light pulse whose duration is approximately equal to the current pulse duration. In active mode locking, the drive current of a laser diode is periodically modulated at a frequency determined by the cavity resonant frequency f of the diode laser, ##EQU1## where c is the speed of light, n is the index of refraction of the lasing material, and L is the physical length of the diode laser. The modulation frequency may be equal to the natural frequency of the diode laser or to some multiple thereof. Mode locking produces pulses which are significantly shorter than pulses produced by current modulation.
Systems based upon gain switching and mode locking need to be compared based upon the key parameters discussed above. Mode locked systems produce shorter, i.e. faster, optical pulses. Shortened pulse duration is related to high performance in applications such as optical-fiber communication. However, mode locked systems require modulation of the current at a precise frequency. On the other hand, gain switched systems produce generally longer pulses, but the frequency of current modulation can be flexibly varied, thereby allowing an easy variation of the repetition rate of the optical pulses.
Gain switched systems are easily synchronized to external timing circuitry while mode locked systems are difficult to synchronize. Such external timing circuitry may, for example, be a system clock in an optical communication system or a device under test in an electrooptic sampling application.
Gain switching allows a straightforward method of modulating a data signal in which a silicon transistor is clocked to an oscillator and it modulates the drive current of the laser. However, drive current modulation by itself is unsatisfactory. The modulation is limited to about 8 ps, and modulation causes the frequency to chirp as the laser begins to lase. Also, silicon transistors incorporated into such current drive circuitry are typically limited to a bit rate of somewhat more than 1 Gb/s. Bottcher et al. have described timing jitter incurred in current modulation in "Detection of pulse to pulse timing jitter in periodically gain-switch semiconductor lasers," Applied Physics Letters, vol. 54, 1989, pp. 1971-1973.
Bowers et al. have developed the theory of mode locking, as introduced above, in "Actively Mode-Locked Semiconductor Lasers," IEEE Journal of Quantum Electronics, vol. 25, 1989, pp. 1426-1439.
Hybrids of gain switching and mode locking offer increased speed. Delfyett, Jr. has described the generation of a train of very short pulses in U.S. Pat. No. 5,265,107 using a laser that is mode locked both by a saturable absorber and by an RF signal applied to the lasing medium at a frequency matched to the mode spacings of the external cavity. Systems relying upon the combination of direct modulation and saturable absorbers are generally limited to transition times of 1 or 2 ps although Delfyett, Jr. provides a somewhat faster system in the previously cited patent.
Recently researchers have recognized the capability of resonant-tunneling diodes (RTDs) for high-frequency oscillation. Capasso et al. have described such devices in High-Speed Semiconductor Devices, (Wiley-Interscience, 1990), ed. S.M. Sze, pp. 465-520, and Grave et al. have described the biasing of these devices in "Monolithic integration of a resonant tunneling diode and a quantum well semiconductor laser," Applied Physics Letters, vol. 58, 1991, pp. 110-112. A basic explanation of their operation is now presented.
The electronic band structure of such a semiconductor device is shown in FIG. 1. An input-side conduction band 10 and an output-side conduction band 12 are physically separated by a tunnel junction 14 including two barriers 16 and an intermediate quantum well 18. The well 18 is so thin that resonant electronic quantum states 22, 24, and 26 are formed in the well 18. In the simple example shown in FIG. 1, both the input and output sides 10 and 12 and the junction 14 are undoped, and it is assumed that donor conduction predominates.
In the unbiased state shown in FIG. 1 or at relatively low biasing voltage, no significant conduction occurs through the junction 14. However, when the junction 14 is electrically biased, as shown in FIG. 2, with the input-side conduction band 10 energetically above the output-side conduction band 12 so that the first resonant state 22 energetically aligns with the input-side conduction band 10, electrons tunnel through the first barrier 16 to the resonant state 22 in the well 18 and then again through the second barrier 16 to above the output side 12. Such alignment provides a high conductivity between the input and output sides 10 and 12, but the conductivity falls on both sides of the alignment so that, on the upper voltage side of the conductivity peak, the differential conductivity or resistivity is negative. Because of the conductivity peak, such devices can form the basis of bistable circuits, as described by Grave et al.
This bistability in resonant tunneling diodes has long been recognized as providing high-speed oscillators, as has been described by Brown et al. in "Oscillations up to 712 GHz in InAs/AlSb resonant-tunneling diodes," Applied Physics Letters, vol. 58, 1991, pp. 2291-2293. Lann et al. in "Phase locking between light pulses and a resonant tunneling diode oscillator," Applied Physics Letters, vol. 62, 1993, pp. 13-15, have described the phase and frequency locking of an RTD oscillator to an external signal source. When the RTD circuit is loaded by a tank circuit, the RTD oscillates freely at some frequency .nu..sub.free =.nu..sub.RTD, where .nu..sub.free is the free oscillation frequency of the RTD and .nu..sub.RTD is the frequency at which the RTD is driven. When driven by an external periodic signal at a frequency .nu..sub.ext, the RTD can adjust is frequency so that ##EQU2## where p and q are positive integers. For example, if p/q=1, there exists a band of frequencies such that when the external signal frequency .nu..sub.ext lies within this band, the RTD adjusts its frequency to maintain .nu..sub.RTD =.nu..sub.ext throughout the band. The width of the band increases with increasing amplitude of the external signal.
When the RTD oscillator and the external signal are locked in the simple relation .nu..sub.RTD =.nu..sub.ext or in the harmonic relationship .nu..sub.RTD =p.multidot..nu..sub.ext or in the subharmonic relationship .nu..sub.RTD =.nu..sub.ext /p, phase locking also occurs. This phase locking causes a constant phase relation between the RTD output and the external signal input to the RTD oscillator. In Lann et al., the precise phase varies as the external frequency .nu..sub.ext is tuned over the locking band.
England et al. have also described the optical switching of resonant tunneling diodes in "Optical switching in a resonant tunneling structure," Applied Physics Letters, vol. 58, 1991, pp. 887-889. Grave et al. in the previously mentioned article have described an RTD switch driving a semiconductor laser.
Nonetheless, the prior art lacks a high-speed oscillator in which the phase of its output relative to its controlling input can be easily controlled.